The invention applies to Mach-Zehnder (MZ) modulators where a high-speed signal is applied to an RF electrode and a separate slowly-varying DC voltage is applied either to the same RF electrode, or to a separate bias electrode. This DC voltage, also called a bias voltage, maintains the bias set point of the interferometer at quadrature, keeping the optical power midway between the full on state and full off state in the absence of an applied RF signal. An AC RF signal applied then swings the optical power either partially or completely from full on to full off, symmetrically about the quadrature point. An additional small AC dither signal of frequency fdither is superimposed on the RF signal, either electronically added or multiplied to the RF signal. The superimposed AC dither signal causes a small optical AC signal to be created in the output of the modulator that can be monitored by a photodetector. The DC bias is adjusted until the fundamental of fdither that appears in the photocurrent is at a maximum. Alternatively, a second harmonic of fdither that appears in the photocurrent can be nulled. The adjustment of the DC bias is provided by a feedback circuit that synchronously detects the AC dither signal and its harmonics in the photocurrent output of the photodetector.
The amount of voltage needed to keep the bias point of the interferometer at quadrature varies with time, temperature, and wavelength, hence a photodetector (to detect optical power) and feedback circuit (to control a voltage) are needed to keep the bias point at the desired point. Note that in some communications applications, the modulator bias point may need to be set at or near full on or full off, or the RF voltage(s) may vary phase of the output light, as well as intensity. In any case, the method of controlling the bias point is similar. One critical aspect of the bias control is the photodetector needed to create a photocurrent or photovoltage that is proportional to either                (1) the optical power of on-state light that is coupled into the output optical fiber, or        (2) the optical power of off-state light that is radiated into the substrate of the modulator.        
Methods of controlling the bias point exist for either scenario. One key problem solved by the invention is the phase tracking error between the transfer curve of the modulator and the transfer curve observed via the photodetector for the latter case, where part of the optical path is unguided in the substrate and/or free-space outside of the substrate.
FIG. 1 shows a conventional prior art Mach-Zehnder (MZ) interferometer modulator 100. An optical signal from an input fiber is coupled into an input optical waveguide 102, in which the lateral field distribution is represented by curve 101. The optical signal is split into two parts 101a, 101b with a y-junction 103a. Each waveguide 104a, 104b following the first y-junction 103a is modulated by a set of electrodes 105a-c in close proximity to the waveguides 104a, 104b. A second y-junction 103b combines the modulated optical signals 101a, 101b. The figure shows electrodes corresponding to a modulator made in x-cut lithium niobate substrate, however, a design for z-cut lithium niobate substrate operates in an analogous manner.
The applied field from the electrodes 105a-c results in a change in the optical phase difference between the modulated optical signals 101a, 101b in the two arms 104a, and 104b of the MZ. If the two modulated optical signals 101a, 101b have a zero optical phase difference, they form a single-lobed guided mode 101c after being combined by the second y-junction 103b into output waveguide 106, resulting in little loss of optical power. If the two modulated optical signals 101a, 101b have an optical phase difference of π (or 180°), then they combine to form a double-lobed higher-order unguided mode 101d that is not supported by the output waveguide 106, causing the light to radiate into the substrate. The radiated light is strongest on both sides of the waveguide 106, approximately into areas 107L and 107R, and weakest in the center, near output waveguide 106. Note that the two lobes have opposite sign in optical field, but have the same intensity.
FIGS. 2(a) and 2(b) show two-dimensional (2D) Beam Propagation Method (BPM) simulations of the MZ operation. The figures show contour plots of the square root of optical intensity (E-field magnitude) for the cases of 0 or π phase difference between the arms of the MZ, respectively, which correspond to on and off switch states of the MZ. Note that in the off-state, the light is radiated primarily in two lobes. The double-lobed beam thus created upon recombination at the output y-junction 103b is rejected by an output fiber coupled to the output waveguide 106. The ripple in the radiation pattern is caused by interference between the radiated light and light radiated elsewhere in the simulation that reflects off the simulation boundary and overlaps the light radiated at the output y-junction 103b. More elaborate 3D BPM simulations show that the off-state radiation lobes not only travel outward, but downward, as well.
FIG. 3 is a graph of intensity vs. drive voltage, for both light intensity in the guided mode 301 reaching the output fiber (dashed line) and the intensity of light radiated into the substrate 302 (solid line). Maximum intensity for light in the output fiber 301a occurs for V=±0.5 Vπ, whereas minimum intensity at the output 301b occurs for V=0V. The curve for intensity of radiated light is the exact opposite, reaching a maximum 302a when light in the output fiber is a minimum and vice versa. The quadrature point 303 is midway between maximum and minimum points along the transfer curves. The dashed plot 301 is referred to as the transfer curve of the MZ, for light output by the MZ. Ideally, the photocurrent in a free-space photodiode is proportional to the light intensity radiated into the substrate, which is represented by the solid curve 302.
FIG. 4a shows a prior art MZ modulator assembly 400 with a free-space photodetector (PD) 407 integrated with a modulator chip 410. The embodiment shown in FIG. 4a is similar to those described in U.S. Pat. Nos. 5,953,466 and 5,963,357. An input light signal is coupled into the MZ input waveguide 402 from input optical fiber 411a held in place on the modulator chip 410 by a transparent fiber block 412a. As before, the light in input waveguide 402 is split into two arms 404a, 404b of the MZ, modulated by electrodes 405a-c and recombined into output waveguide 406. The output waveguide 406 is coupled into an output optical fiber 411b, attached to the modulator chip 410 by a transparent fiber block 412b. The PD 407, located underneath the output optical fiber 411b collects light that is radiated into the substrate, after it passes through the transparent fiber block 412b. The PD 407 may or may not be biased with a voltage across it. The light illuminating the PD 407 causes a photocurrent to be generated. The PD 407 is connected to an electrical circuit such as a transimpedance amplifier or op amp circuit that amplifies the photocurrent, converting it into a voltage. Typically, the electrical circuit is external to the modulator.
FIG. 4b is a side view of the prior art MZ modulator assembly 400 shown in FIG. 4a. The dashed arrows illustrate the path followed by off-state light emitted from the output waveguide 406 on the modulator chip 410 passing through substrate 410a at an acute angle slanting away from the top surface of the modulator chip 410 and through the transparent fiber block 412b to the PD 407 located underneath the output optical fiber 411b. 
FIG. 4c shows a close-up of the transparent fiber block 412b, output optical fiber 411b, and PD 407. The arrows represent light radiated from different locations of the modulator (not visible in this view). The thick solid line arrows represent the off-state light radiated into the substrate when the MZ is switched off. There are additional components of radiated light coming from the modulator due to optical loss of various structures in the device. The thick dashed line arrows represent on-state light lost in the MZ output y-junction even when the MZ is switched fully on. This radiated on-state light is an amount of light radiated into the substrate that is proportional to the light launched into the output optical fiber 411b. 
The thin dotted line arrows represent another component of radiated on-state light, coming from the junction of the modulator output optical waveguide and the output optical fiber. In general, this component of radiated on-state light is much larger than the component coming from y-junctions and other features along the modulator, however is comparable to the amount of light radiated into the substrate at the junction of the input optical fiber and the modulator input optical waveguide. Note that these on-state light components are strongest along the direction of the fiber, and are weaker for more diverging angles. On the other hand, the off-state light components are strongest in the diverging angle direction and weak along the direction of the output fiber. In prior art modulators, the on-state light also reaches the photodetector, creating an interference pattern that depends on the drive voltage.
FIG. 4d shows another example of a prior art MZ modulator assembly 430 with a free-space PD 407 using a mirror or mirrored surface 415 to reflect the light and direct it to the PD 407. The embodiment shown in FIG. 4d is similar to designs described in U.S. Pat. No. 7,200,289, in which off-state light captured by an output fiber block is deflected to a photodetector. The back of the output fiber block may have an oblique angle and may be reflective. The addition of the mirror 415 allows the PD 407 to be positioned in a location that is closer to a shelf within the assembly 430, allowing a simpler means of electrical connection. The surface of the mirror 415 may be polished or roughened. A rough surface causes the reflected light to be diffused to a larger degree, which may be desirable, if the photodetector is positioned far away from the output optical fiber or pigtail 411b. A more diffuse reflection increases the possibility of multiple reflections, causing the reflected light to travel farther. Hence, a diffuse reflection simplifies the choice of photodetector location, as the entire cavity of the modulator package is illuminated by the reflected light.
FIG. 5 is a plot of the detected signal in relative units vs. normalized drive voltage on a MZ modulator. The solid line 502 is the ideal curve, which is proportional to the radiated off-state light 302 plotted in FIG. 3. The dashed curve 504 shows the detected signal for the case where the magnitude of on-state light is significant compared to the off-state light. Note that the dashed curve 504 not only shows a low on/off ratio, given by the ratio of its maximum to minimum values, but is also shifted laterally along the voltage axis. The amount of lateral shift from ideal is referred to as the PD phase tracking error.
The feedback circuit controlling bias voltage sets the bias point to quadrature point of the PD transfer curve, which is shifted slightly from the true quadrature point of the transfer curve of optical power transmitted to the output optical fiber. Hence, there is an error in setting the bias voltage, which results in degradation in system performance. There is some also some impairment due to the reduction of on/off ratio, namely reduced gain in the control loop, and some additional noise, however, these impairments can be largely overcome by proper design of the feedback control circuit.
The limitations of prior art free-space integrated photodetector designs can be overcome by understanding the cause of the tracking error in more detail. The E-field of radiated light on the left and right sides of the output waveguide are given by Equations 1 and 2, respectively.EA=A01 cos(θ)+j A11(cos(φ)+j sin(φ))sin(θ)  (1)EB=A02 cos(θ)−j A12(cos(φ)+j sin(φ))sin(θ)  (2)
The coefficients A01 and A02 represent the field strength of on-state light reaching the photodetector at the two locations, labeled 107L and 107R in FIGS. 1, 2a and 2b, respectively, whereas the coefficients A11 and A12 represent the field strength of off-state light at those locations. The symbol ‘j’ represents the imaginary unit, which equals the square root of −1. The strength of on-state and off-state light is represented by cos(θ) and sin(θ), respectively, due to the interference effect produced by the MZ. The cos(φ)+j sin(φ) term accounts for the unknown phase relationship between on-state and off-state light that can change as a function of location, wavelength, and/or temperature. Note that the E-field polarity of on-state light has the same sign on both sides of the output waveguide due to symmetry, while the E-field polarity of off-state light is intrinsically different in sign, due to the anti-symmetry of the first higher-order mode. The intensity of the light reaching the photodetector at the two locations is given byIA=A012 cos2(θ)+A112 sin2(θ)−2 A01 A11 sin(θ)cos(θ)sin(θ)  (3)IB=A022 cos2(θ)+A122 sin2(θ)+2A02 A12 sin(φ)cos(θ)sin(θ)  (4)
The last term of Equations 3 and 4 is caused by the coherent interference between on-state and off-state light, and is the cause of phase tracking error. The total photocurrent, assuming the photodetector covers locations L and R, is given byitotal=(RAA012+RBA022)cos2(θ)+(RAA112+RBA122)sin2(θ)+2(RBA02A12−RAA01A11)sin(φ)cos(θ)sin(θ),  (5)where RA and RB are the photodetector responsivity at the two locations. The last term in Equation 5 is responsible for phase tracking error. Note that it equals zero whenRBA02A12=RAA01A11.  (6)If responsivities, field strengths of on-state light, and field strength of off-state light are exactly the same for the two locations, then Equation 6 is satisfied, that isRB=RA A01=A02 A11=A12  (7)
FIG. 4e shows a prior art design with two photodetectors 407a and 407b, where Equation 7 is valid, allowing the simple circuits shown in FIGS. 6a, 6b to be used. Each photodetector 407a, 407b collects one lobe of the radiated off-state light in equal proportion. The photodiodes are connected either in series (FIG. 6a) or parallel (FIG. 6b) in order to sum the photocurrents equally, requiring the net responsivities, RA and RB, to be equal.
The use of common lens to couple light from an output waveguide to a fiber and couple off-state light from a substrate to photodetector surface is shown in Itou (U.S. Pat. No. 5,764,400).
Hosoi (U.S. Pat. No. 6,668,103) describes various ways of deflecting light from one output port of a 3 dB coupler to a monitor photodiode. The path between waveguide output and photodiode is shown as propagating through free-space. Various ways of deflecting light to a photodiode integrated on the substrate are also shown. Part of the optical path may include free-space within the substrate.
Vaerewyck (U.S. Pat. No. 4,768,848) discloses a device having an optical tap coupler on the input waveguide. The light from the tap is directed to a photodetector, while Okada (U.S. Pat. No. 5,111,518) describes a device with a folded waveguide to carry light to a photodetector.
Isono (U.S. Pat. No. 5,259,044) describes a device with a folded optical path where tapped light or Fresnel reflected light is reflected and guided to a photodetector, with a portion of the optical path appears to propagate free-space within the substrate.
In practice, it is difficult to match the field strengths in the two locations. Also, in reality Equation 5 needs to be integrated over the entire surface area of the photodetector, hence all of the parameters are likely to vary somewhat with spatial location, making it more difficult to cancel out the net tracking error term. The strength of the E-fields may also vary with temperature and wavelength, making cancellation of the phase tracking error term more problematic.
It is an object of the present invention to address these difficulties by presenting a MZ modulator structure with light-blocking material on or within an output fiber block and/or around an output optical fiber to block on-state light, and possibly one lobe of off-state light, thereby preventing it from reaching one or more photodiodes used to monitor drift in Mach-Zehnder interferometer modulator.